1. Field of the Invention
The present invention relates to a high frequency variable gain amplification device, a control device, a high frequency variable gain frequency-conversion device, and a communication device. More particularly, the present invention relates to a high frequency variable gain amplification device, a control device, a high frequency variable gain frequency-conversion device, and a communication device which are for use in preventing a distortion characteristic from being reduced during an operation with low gain.
2. Description of the Background Art
In a radio communication system including a receiver unit typified by a mobile telephone, in order for the receiver unit to receive a weak desired signal, a first stage amplifier is required to have a low noise characteristic and a high gain characteristic. Alternatively, in order for the receiver unit to receive a large desired signal, the first stage amplifier is required to have a low gain characteristic and a low distortion characteristic. Particularly, in recent mobile communication, field intensity at reception may vary considerably depending on a distance between a base station and a terminal device. Accordingly, the receiver unit is required to have a large dynamic range. Therefore, a low noise amplifier at a reception front end portion is required to have a gain control function.
In an amplification device having the gain control function, current consumption is increased/decreased so as to cause gain to vary (see, for example, Japanese Laid-Open Patent Publication No. 2002-016462).
FIG. 17 is a diagram showing a structure of a conventional reception circuit 900 having a gain control function which is disclosed in Japanese Laid-Open Patent Publication No. 2002-016462. As shown in FIG. 17, the conventional reception circuit 900 includes an amplification element 901, constant impedance elements 902 and 903, and a variable resistor 904 connected in parallel with the constant impedance element 902. If a level of a received signal is excessively high, the reception circuit 900 increases a resistance value of the variable resistor 904 to reduce current consumption, thereby reducing gain. This reduces an output power for the same input power, resulting in a reduction of third-order intermodulation distortion (IM3).
A similar conventional technique is disclosed in Japanese National Phase PCT Laid-Open Patent Publication No. 2002-536859.
As described above, a conventional reception circuit reduces current consumption in order to reduce IM3. However, the reduction of current consumption reduces the gain of the reception circuit, so that gain of a desired signal is also reduced, resulting in a reduction of a distortion characteristic, e.g., third-order intercept point (IP3), which is obtained by relatively comparing IM3 with the desired signal.
FIGS. 18A and 18B are diagrams used for explaining how third-order output intercept point (OIP3) decreases with reduction of current consumption in a conventional reception circuit. As is shown by transition from point a to point b in FIG. 18A, in the conventional reception circuit, gain is reduced by reducing current consumption. However, as is shown by transition from point a to point b in FIG. 18B, mere reduction of the current consumption results in a reduction of OIP3. Accordingly, third-order distortion increases with an input power level, so that the third-order distortion becomes dominant. As described above, the conventional reception circuit has a problem that reduction of current consumption might reduce a distortion characteristic together with gain. Reducing current consumption is not compatible with preventing the distortion characteristic from being reduced.
A high frequency variable gain frequency-conversion device including an amplification element has a problem similar to the problem as described above.